This invention relates to a perpendicular magnetic recording medium to be loaded in a hard disk drive (HDD) of a perpendicular magnetic recording system.
In recent years, following the advancement of the information society, various types of information processing apparatuses have been proposed. Further, information recording apparatuses for use in those information processing apparatuses have been proposed also. In order to achieve a smaller size and a higher performance of the information processing apparatuses, the information recording apparatuses are required to have a greater information recording capacity and a higher recording density.
As one of the information recording apparatuses, there is known a magnetic recording apparatus, such as a hard disk drive (HDD), using a magnetic disk as a recording medium. In the magnetic recording apparatus, it is desired to achieve a large information recording capacity, for example, greater than 30 GB (gigabyte) per a 2.5-inch magnetic disk.
In order to increase the information recording capacity of the magnetic disk, it is necessary to improve both of a performance of the magnetic disk and a performance of a magnetic head for recording and reproducing an information signal to and from the magnetic disk. In order to meet the above-mentioned demand, the magnetic disk is required to achieve an information recording surface density greater than 60 Gbit/inch2.
In the magnetic recording apparatus widely used at present, the magnetic disk has a magnetic recording layer of a so-called in-plane magnetic recording system (a longitudinal magnetic recording system or a horizontal magnetic recording system). In the in-plane magnetic recording system, the magnetic recording layer has a magnetization direction substantially parallel to a principal surface of the magnetic disk.
However, the in-plane magnetic recording system is disadvantageous in the following respect. Specifically, in case where crystal grains in the magnetic recording layer are reduced in size in order to carry out information recording at a high recording surface density such as 60 Gbit/inch2, an influence of a demagnetizing field between neighboring crystal grains is increased so that a good recording quality may not be achieved. Further, in case where the crystal grains in the magnetic recording layer are reduced in size, the magnetic recording layer must be reduced in thickness. This results in easy occurrence of a thermal fluctuation error due to thermal magnetic aftereffect. If the thermal fluctuation error is significant, recording magnetization is attenuated with lapse of time. Eventually, recorded information can not normally be reproduced.
Under the circumstances, it is recently proposed to adopt a perpendicular magnetic recording system in the magnetic disk instead of the in-plane magnetic recording system. In the perpendicular magnetic recording system, resistance against the thermal fluctuation error is high even if the recording surface density is increased. Thus, the perpendicular magnetic recording system is a recording/reproducing system favorable for achievement of information recording at a high recording surface density.
In order to adopt the perpendicular magnetic recording system in the magnetic disk instead of the in-plane magnetic recording system, it is necessary to substantially change a structure of the recording layer. Specifically, in the magnetic disk adopting the perpendicular magnetic recording system (will hereinafter be called a “perpendicular magnetic recording disk”), an easy magnetization axis of the recording layer as a hard magnetic layer must be oriented in a perpendicular direction (normal-line direction) with respect to a principal surface of the magnetic disk.
For example, in case where the recording layer is made of a cobalt (Co) based ferromagnetic material, the easy magnetization axis of the recording layer is given by a c axis of a hexagonal close packed (hcp) crystal structure of cobalt. Therefore, in this case, the c axis of the crystal structure of cobalt must be oriented in the perpendicular direction with respect to the principal surface of the magnetic disk.
To this end, in the perpendicular magnetic recording disk, it is necessary to provide a nonmagnetic underlayer for promoting perpendicular orientation of the easy magnetization axis of the recording layer and to form the nonmagnetic underlayer and the recording layer by epitaxial growth (heteroepitaxial growth).
For example, Japanese Patent Application Publication (JP-A) No. 2003-77122 (corresponding to U.S. patent application Publication No. 2003-064253) discloses a perpendicular magnetic recording disk comprising a disk substrate of a nonmagnetic material, a seed layer (first underlayer) of a metal or an alloy having a face-centered cubic (fcc) crystal structure, a nonmagnetic underlayer (second underlayer) formed on the seed layer and made of a metal or an alloy having a hexagonal close packed (hcp) crystal structure, and a magnetic layer as a recording layer formed on the nonmagnetic underlayer.
Japanese Patent Application Publication (JP-A) No. 2002-92865 discloses a perpendicular magnetic recording disk comprising a disk substrate of a nonmagnetic material, a first underlayer formed on the disk substrate and made of titanium (Ti) or an alloy containing titanium, a second underlayer formed on the first underlayer and containing chromium (Cr), and a magnetic layer as a recording layer formed on the second underlayer.
Japanese Patent Application Publication (JP-A) No. 2002-74648 (corresponding to U.S. patent application Publication No. 2002-048693) discloses a perpendicular magnetic recording disk comprising a disk substrate of a nonmagnetic material, a soft magnetic layer formed on the disk substrate and made of ferromagnetic nanocrystals, an intermediate layer formed on the soft magnetic layer and containing nickel (Ni) as a main component and zirconium (Zr), and a magnetic layer as a recording layer formed on the intermediate layer.
Recently, the magnetic disk is required to have an information recording surface density of 100 Gbit/inch2 or more. In order to achieve a perpendicular magnetic recording disk capable of carrying out information recording and reproduction in good condition at such a high recording surface density, it is necessary to more strictly control the perpendicular orientation of the easy magnetization axis of the recording layer so as to assure a predetermined S/N ratio and a predetermined resolution.
On the other hand, it is believed that the perpendicular magnetic recording disk is preferably a so-called double-layer perpendicular magnetic recording disk comprising a soft magnetic layer formed on a disk substrate and made of a soft magnetic material or ferromagnetic nanocrystals and a recording layer formed on the soft magnetic layer and made of a hard magnetic material, as described in Japanese Patent Publication Application No. 2002-74648. This is because, in the double-layer perpendicular magnetic recording disk, a suitable magnetic circuit can be formed through a magnetic head, the recording layer, and the soft magnetic layer during magnetic recording. Based on a mirror-image effect, the soft magnetic layer exhibits a function of assisting the magnetic recording. Thus, it is believed that the soft magnetic layer being formed between the disk substrate and the recording layer is a structure preferable as the perpendicular magnetic recording disk.
Disadvantageously, however, formation of the soft magnetic layer inhibits the perpendicular orientation of the recording layer. Even if the nonmagnetic underlayer is formed between the soft magnetic layer and the recording layer, the soft magnetic layer disturbs the orientation and a surface characteristic of the nonmagnetic underlayer. In this event, a desired effect of the nonmagnetic underlayer, i.e., the effect of improving the perpendicular orientation of the recording layer may not be exhibited.
In case where the structure of the recording layer of the magnetic disk is substantially changed in order to adopt the perpendicular magnetic recording system instead of the in-plane magnetic recording system as described above, a development cost and a production cost are drastically increased as compared with the magnetic disk of the in-plane magnetic recording system already widespread in the market. Therefore, an inexpensive magnetic disk may not be supplied. Further, if it is necessary to more strictly control the perpendicular orientation during deposition of the recording layer as described above, the production cost of the perpendicular magnetic recording disk will further be increased.